The different types of document printing are divided into two groups: one known as “static”, in which each document receives the same printed mark, for example an “offset” analog print process, and the second known as “serialized” digital, in which each document receives an individualized item of information, for example an ink-jet print process controlled by an individualization program, and a process for printing a serial number.
For offset printing, which is one of the most commonly used print methods for boxes and packaging, a plate is generated for each color printed in the document, and this plate's content is printed hundreds of thousands, even millions, of times. In this case, the same content, inserted on the printing plate, is printed on every document for every print. Flexography, typography and gravure printing are other examples of what are called static printing methods. In static printing, documents cannot be identified individually, in theory, since the same mark is printed each time. In addition, when the printing is static and makes use of analog processes, it is more difficult to control the exact number of documents printed. The risks of counterfeiting through printing a larger quantity of documents than the owner of the rights has authorized are therefore significant. How can you ensure that the number of prints specified by the manufacturing order, often less than the plate's usage limit, has been respected? How can you ensure that all the unused prints (start or end of the series, faults, order cancelled, etc) and all the plates, films and other objects that allow the documents to be reconstituted never fall into the hands of counterfeiters?
Serialized printing, by allowing each document to be precisely and unequivocally identified, is generally preferable to static printing. In effect, each identifier being only printed once in serialized printing, reading a double means that an alarm can be triggered: a double is an identifier that is identical to a previously read identifier.
In a general way, there are several points to be made secure in order to protect identifier and/or anti-copying marks: the source file, possibly the CAP file that contains it, and, in the case of offset printing, the plates and any films.
It is possible to perform the equivalent of serialized printing of an anti-copying mark on an item already printed statically by, in a second step, printing a unique code or serial number that is uncoded or, preferably, encrypted. This serialized printing can, for example, take the form of a two-dimensional bar code. Outwardly, this procedure makes it possible to track each document individually and at the same time retain a sure way of detecting copies. Stolen documents that have not received the serialized print would not bear a valid identifier.
This approach does not, however, solve all the problems. In effect, while a wrongdoer cannot identify the falsified documents as the printer would have done, the unique code printed by the serialization printer, generally offering a limited print quality, is not protected against copying.
Counterfeiters having in their possession documents to be identified as authentic can therefore copy one or more valid unique codes and re-copy them onto documents to be identified as authentic.
The prior state of the art contains several methods exploiting measurable physical characteristics in order to characterize and identify each document in a unique way. In general, the measurable physical characteristics chosen are of a random nature, and according to the actual state of the art and technologies cannot be copied, at least not in a cost-effective way. These methods enables all the documents considered “valid” to be controlled: only those documents for which the physical characteristics, comprising a unique set, have been memorized are considered valid.
For example, U.S. Pat. No. 4,423,415 describes a method enabling a sheet of paper to be identified according to its local transparency characteristics. Several other procedures are based on inputting unique and non-reproducible physical attributes of the material in order to generate a unique and non-transferable signature of said document. For example, documents WO 2006 016114 and US 2006/104103 are based on the measurement of the diffraction pattern induced by a laser ray applied to a precise area of the object.
Although they offer an interesting solution to the problems mentioned above, the approaches based on extracting a signature from the material are difficult to use for a number of reasons. Firstly, recording signatures when the documents are produced requires a costly optical reader and is difficult to integrate into production lines. These latter may, moreover, have very high working speeds. In a general way, it seems that these techniques are only applicable to small-scale production. In addition, the reader used for checking, in the field, is also too costly for a number of applications. It is also bulky and not easy to use, while often the checks in the field must be done rapidly and unobtrusively. Finally, it is not possible to extract a unique signature for all materials: glass and objects that are too reflective are excluded, in particular, at least for measurements of a laser's diffraction.